1. Field of the Invention
The present invention relates generally to apparatuses and methods for transmitting and receiving in a multi-antenna system, and more particularly, to a transmitter that precodes and then transmits a transmission signal on an n-th frame and a complex conjugate signal of the transmission signal on an (n+1)-th frame, and a receiver that divides a restore matrix for post-processing into a real part and an imaginary part, and then performs the post-processing.
2. Description of the Related Art
Using a plurality of transmission and reception antennas, a multi-antenna system can enhance channel capacity and transmission and reception performances without additional frequency assignment or increasing transmission power. The multi-antenna technology includes (1) a spatial diversity based scheme that obtains a diversity gain corresponding to a product of numbers of the transmission antennas and the reception antennas, and raises the transmission reliability using symbols passing through various channel paths by simultaneously transmitting the same data, (2) a spatial multiplexing scheme that raises the data rate by simultaneously transmitting multiple data symbols, and (3) a combination of the two schemes.
To separate the transmission signals of a plurality of user terminals in the multi-antenna system, a channel matrix estimated at a receiver of each individual user terminal is fed back to a transmitter of a base station. The transmitter of the base station precodes and transmits the signal using the fed back channel matrix of the user terminal. Because the feedback information is restricted by resource limitation of the uplink, the receiver feeds back an index of a codebook by mapping the channel matrix to feed back to an appropriate codeword using the codebook designed based on the channel matrix.
For example, in a 4×2 zero-forcing beamforming system using quantized channel information, each user terminal determines a reception beamforming weight we through a downlink channel matrix Hk, maps an effective channel hkeff=wkHk to one codeword of the codebook C={c1, c2, . . . , cL}, and feeds back the index of the corresponding codeword. The base station selects users for the data transmission through scheduling and precodes with an inverse matrix of the matrix including the codewords fed back from the selected users {k1, k2, k3, k4}. The precoding matrix is given as shown in Equation (1).
                    P        =                              [                                                                                c                                          k                      1                                        T                                                                                                                    c                                          k                      2                                        T                                                                                                                    c                                          k                      3                                        T                                                                                                                    c                                          k                      4                                        T                                                                        ]                                -            1                                              (        1        )            
In Equation (1), P denotes a preceding matrix, ck1T denotes a codeword of a user terminal k1, ck2T denotes a codeword of a user terminal k2, ck3T denotes a codeword of a user terminal k3, ck4T denotes a codeword of a user terminal k4, and (•)T denotes a transposed matrix.
The preceding matrix P, which includes the quantization error, is subject to interference between the multiple user terminals.
When the codeword index of the pre-designed codebook is fed back based on the channel matrix estimated at the receiver and a Grassmannian codebook is used, the bit error is greater than 0.1 and an error floor value exhibiting no performance change, even when a Signal to Noise Ratio (SNR) increases, is considerable. When there is no limitation on the feedback, the receiver may quantize and feed back the elements of the channel matrix. Consequently, the error floor, which is about 0.02, is lower than when the Grassmannian codebook is used. However, to feed back each element of the channel matrix of the user terminal with certain bits, the feedback is considerable and the computations required for the quantization increase.